This invention relates to preparing a cationic polyvinyl alcohol additive suitable for addition in the wet-end of a paper making process and more specifically to a non-formaldehyde additive which is a stable fluid aqueous solution at high solids yet imparts improved performance to the resultant paper.
There are numerous commercial wet end additives being used in paper making. Among the additives are cationic starches and melamine-formaldehyde resins.
Cationic starches are used for improving retention of cellulosic fines, filler and pigment, and also for increasing the dry strength of the resulting paper. However, the use of cationic starch can lead to irregularities in performance (irreproducibility of batches, low solution stability, low wet strength), incompatibility with other components in the furnish (alum, size, other salts), and high biological oxygen demand (BOD) for additive not on the pulp or recycled, and which is lost in the waste water.
Other wet-end additives are often used to confer permanent wet strength to the resulting paper, such as cationic urea-formaldehyde UF resins, amine-containing polyamides treated with epoxides (e.g. Hercules` "Kymene" 557) or melamine-formaldehyde (MF) resins (e.g., "Parez" 607 of American Cyanamid). However, UF resins are slow curing on the machine, while the polyamides are relatively expensive, slow to absorb on the cellulose pulp, and make repulping of the paper relatively difficult. The MF resins show poor pigment and filler retention, and also exhibit low water absorbency, whereas absorbency is often desired along with wet strength. All of these types of additives give only modest enhancement of dry strength. Also, none of the above types are now recognized as improvers of wet web strength (at their usual concentration of application) which would permit greater production control and in some cases, increased productivity.
Polyvinyl alcohol has been used for surface sizing and surface coating because of its excellent film forming and binder characteristics. However, polyvinyl alcohol is not added directly to the wet-end of the paper making process because most of the polyvinyl alcohol would pass through the wet paper web end into the white water. The absorption and retention of unmodified alcohol on fiber and filler is insufficient to resist the voluminous water discharge and the hydrodynamic forces present during paper formation.
Methods have been disclosed for associating polyvinyl alcohol with a positive charge to allow it to be incorporated into the wet-end of paper making by reacting the polyvinyl alcohol with trimethylolmelamine acid colloid (see Tappi Journal, Volume 66, No. 11, 11/1983). Also U.S. Pat. No. 4,461,858 discloses a polyvinylalcohol/melamine-formaldehyde resin interaction product. However a serious drawback in the use of such materials is that they contain free formaldehyde. The presence of free formaldehyde is undesirable, not only because of its unpleasant odor, but because it is an allergen and an irritant, which can cause severe reactions in the operators who manufacture the agent and who treat and handle products containing it. A further drawback for the cationic polyvinyl alcohol/melamine-formaldehyde resin products as disclosed in U.S. Pat. No. 4,461,858 is that the solutions are stable only at very dilute concentrations (e.g., 3%) and gel at higher solids (e.g., gel in minutes at 12% solids). Another drawback of U.S. Pat. No. 4,461,858 is that in forming the MF resin acid colloid at a pH near 1 to 2 concentrated hydrochloric acid is used, which is a very corrosive solution which requires exotic materials for proper handling.
A further problem of the prior art is that the amount of cationic charge imparted to the polyvinyl alcohol is difficult to control as the melamine-formaldehyde resin acid colloid is a variable composition. In addition, the use of melamine-formaldehyde resins or polyamino polyamide epichlorohydrin adduct as a wet-end additive makes the resultant paper difficult to repulp and recycle because the resin does not readily break down during repulping.